The main industrial process for producing nitric acid is known as the Ostwald route where ammonia, NH3 is oxidised over a combustion catalyst at temperatures in the range of 800 to 1100° C. to form nitrogen monoxide, NO. The pressure ranges from atmospheric pressure to 10-12 bar. The formed nitrogen monoxide is quenched, mixed with air to form nitrogen dioxide, NO2, and then the nitrogen dioxide is allowed to react with water to form nitric acid, HNO3.
The typical combustion catalyst is one or more stacked gauzes made of woven or knitted wires of platinum alloyed with rhodium, and traces of grain refining elements. During operation the combustion catalyst loses platinum and to a lesser extent rhodium via the volatisation of PtO2 and RhO2. Thus it is common industrial practise to place catchment gauzes downstream of the combustion gauzes in order to recover a proportion of the platinum loss.